Modern machine tools are becoming increasingly automated and faster in operation in order to more precisely and quickly process a workpiece. Examples of such machine tools are boring machines, milling machines, drilling machines, and machining centers, which combine into a single unit a boring machine, milling machine, and a drilling machine.
A boring machine operates to enlarge (to specific dimensions) the size of an existing hole in a workpiece. A milling machine rotates suitable milling cutters to enable complicated cutting operations on a workpiece including planar cutting, groove cutting, general straight cutting, etc. A drilling machine mounts a cutting tool such as a drill in a rotating spindle, and the spindle is rotated then lowered and raised to form a hole in the workpiece.
A machining center is a machine tool that allows for multiple axis processing and multiple production processing through rectilinear, rotational, and spindle rotational motion, and by only a single setting. In a numerical control machine tool, which is another name for a machining center, movement of a cutting tool or the number of rotations of a spindle is pre-recorded on a punch tape as needed (according to a particular design). The punch tape is supplied to a control unit that operates to precisely and automatically realize a complicated workpiece design. The punch tape may be used for subsequent workpieces so that multiple, identical workpieces may be formed.
In such machine tools, typically a suitable tool is mounted in a spindle that is rotated by a motor (typically referred to as a spindle motor), and the rotating tool processes a workpiece. Generally, a spindle is mounted within a spindle housing and a tool having a concentric catch protrusion is received and secured in an end of the spindle. A collet securely holds and releases the catch protrusion of the tool. A plurality of bearings are interposed between a front end portion and a rear end portion of the spindle and the spindle housing. The bearings allow rotation of the spindle with respect to the spindle housing. Also, to support a rear end portion of the spindle, a needle bearing may be used. Lubricant may be supplied through a lubrication hole formed in the support structure around the needle bearing. In order to permit forward and reverse motion as well as rotational motion of the spindle, there are gaps associated with the bearings, in particular the needle bearing.
However, a number of drawbacks exist in conventional spindle assemblies. For example, if the rotational speed of the spindle is increased for high-speed operation, the bearing gap causes the generation of rotational vibration such that the precision in the processes performed on the workpiece is reduced and/or noise is generated. It also becomes necessary that the processes be performed in accordance with the supply of a lubricant for needle bearing lubrication and cooling.